There are many applications where multi-cell batteries with alkali electrolytes are used and where the ambient conditions involve temperature changes and thermal shock exposures between wide extremes of low temperature and of elevated temperature with a high humidity condition. Under such conditions, it has been found necessary to house such batteries in metallic outer containers, since at low temperatures plastics may become quite brittle and can be easily cracked if subjected to any sort of physical shock.
The metal comprising the outer container must possess certain properties, though. It should be non-magnetic, non-corrosive, light in weight but of high strength. While certain forms of steel or plated steel may be used for the housing, it has been found that steel and even plated steel can corrode and rust under thermal shock conditions, thereby leading to chips, cracks and pits in the container. Aluminum, however, was found to possess all the necessary properties to enable the housing to withstand thermal shock conditions, and thus is the preferred housing material herein.
When alkaline electrolyte batteries are exposed to thermal shocks, large quantities of electrolyte may be released during a venting operation. As a result, discharge of liquid electrolyte from interstack cells will occur, leading to external battery leakage.
Although protective encapsulating materials such as epoxy are used to cover the cells and to enclose them, such protection is insufficient to prevent leakage under extreme temperature variations. At elevated temperatures, the encapsulating material seems to become sufficiently porous to accept and permit seepage of the electrolyte and of surface moisture which remains when the ambient temperature declines. As the temperature continues to drop, the moisture and seepage filaments may be converted to the frozen phase, so that even minute quantities of moisture or seepage can create internal fracture conditions due to expansion upon freezing.
Further, under extreme temperature variations, the individual cell components, such as the seal and the container, will expand and contract to a greater degree than the surrounding encapsulating material. The net result will be spaces adjacent the external cell container which will receive any released liquid electrolyte, which electrolyte will then find its way to the enlarged fissures and internal crevices formed in the protective enclosing material.
Sealing of individual electric cells has been developed to a very high degree of effectiveness. Nevertheless, because of slight variations that may occur during the manufacturing processes, some conditions do occasionally occur that also permit leakage from an apparently well-sealed cell. Such leakage permits the electrolyte to exude from the cell and such electrolyte also may find its way to the enlarged fissures and internal crevices formed in any protective enclosing material.
When a cell or stack of cells is enclosed in an aluminum outer case, the alkali electrolyte attacks such an aluminum housing and detrimentally affects the battery. Even worse, if the electrolyte manages to eat away at the housing and to pass beyond the housing, the alkali may attack the components of the circuitry, with which a battery is assembled, as in electronic equipment.
The object of this invention is to provide a simple construction for a battery stack, for a small battery, and for an assemblage of several battery stacks in a larger battery, which will serve to isolate the cells of the stack in such manner as to prevent the exudation of cell electrolyte from establishing short circuits by intercell bridging, or by interstack bridging, or from exiting from the batteries and corroding associated electronic equipment.
Another object of the invention is to provide an assembly combination in which a simple and inexpensive moisture-proof cup is utilized as a container for a stack of electric cells together with a body of enclosing epoxy resin which are assembled in such manner as to enable the plastic cup to serve as a moisture barrier and enclosing shield.
Another object of this invention is to provide a novel assembly construction for the elements of the battery, which will prevent or at least suppress the harmful effects from the moisture in a highly humid atmosphere to which the batteries may be subjected with their associated equipment for relatively long periods of time.
Another object of the invention is to provide a combination assembly in which a plastic cup is utilized as a primary container and confining structure for a stack of superposed electric cells and a body of epoxy resin. The resin serves to engage and enclose the cells and to fill all the space surrounding and in between the cells of the stack, defined within a predetermined volume of space, so the epoxy resin upon changing from its poured fluid state to its solid state will provide a totally enclosing structure around the cells of the stack while individually isolating the cells. In the aggregate, the resin provides a solid encapsulating enclosure for the stack, which in turn is essentially moisture sealed by the plastic cup to prevent exiting of any electrolyte outwardly from the stack space to adjacent electronic equipment, and to prevent the entrance of ambient moisture to the encapsulating material while such material is being subjected to cycles of temperature change.
Another object of this invention is to provide an effective heat seal and moisture barrier around each encapsulating mass surrounding each stack.